橋梁專業(yè)畢設(shè)文獻(xiàn)翻譯知識分享

1、NANJING UN I VEH SI TV OF TECHNOLOGY畢業(yè)設(shè)計（文獻(xiàn)翻譯）譯文及原文復(fù)印件學(xué)生姓名：左燁學(xué) 號：1804070233所在學(xué)院：交通學(xué)院專業(yè)：交通工程文獻(xiàn)題目：Bridges橋梁類型指導(dǎo)教師：羅韌南京工業(yè)大學(xué)土木工程學(xué)院交通工程系二O 一一年三月橋梁類型梁橋梁橋也許是最普遍也是最基本的橋梁結(jié)構(gòu)形式。 一根木頭跨越小河是典型梁 橋的一種最簡單的實例。在現(xiàn)代的鋼梁橋結(jié)構(gòu)形式中，最常見的兩種型式是 I 型梁橋和箱梁。如果我們考察 I 型梁的橫截面我們馬上就能理解為什么它被冠以如此的名 字（見插圖 1）。梁的橫斷面采用了英文字母 I 大寫的形狀。中間的垂直板被稱 為肋板

2、，而頂部和底部的平面板指的就是凸緣。 要解釋 I 型梁為什么是一種有效 的截面型式是一項長期而艱巨的任務(wù)， 因此在本篇文章中我們就不做解釋了。 箱 梁得名如同于 I 型梁一樣，很明顯， 它的截面形狀類似于一個箱子。 典型的箱梁 截面型式有兩個肋板和兩個凸緣（見插圖 2）。但是在某些情況下，兩個以上的肋板就會形成存在剪力滯現(xiàn)象的多室箱梁。其他梁橋的例子包括n型梁一一因其截面類似于數(shù)學(xué)符號n而得名，還有T型梁。因為絕大部分現(xiàn)今建造的梁橋都屬于箱梁或是 I 型梁，所以我們會跳過這 些少見的截面類型?，F(xiàn)在我們了解了 I 型梁和箱梁之間的外形差異， 讓我們來看看這兩種截面型 式的優(yōu)缺點。 I 型梁截面設(shè)

3、計和建造簡單，在大部分情況下，使用效果也很好。 但是，如果梁橋有任何形式的彎曲，梁就會受扭，也就是眾所周知的扭矩。對比 于I型梁，在箱梁中添加的第二個肋板增加了穩(wěn)定性， 也就增加了箱梁的抗扭性 能。這就使得箱梁截面成為存在顯著曲線梁橋的理想選擇。箱梁結(jié)構(gòu)更穩(wěn)定可以跨越更長的距離且經(jīng)常用于大跨徑橋梁， 而使用 I 型梁 就不會有足夠的強(qiáng)度和穩(wěn)定性。然而，設(shè)計構(gòu)造箱梁橋相比于 I 型梁更為困難。 例如，為了焊接箱型梁內(nèi)的接縫， 人工或是機(jī)械就必須能夠控制箱梁產(chǎn)生的剪力 滯。桁架橋桁架是一種簡單的類似于骨骼的結(jié)構(gòu)。 在結(jié)構(gòu)力學(xué)中， 簡單桁架的單個組成 部分只受到拉力和壓縮力，而不存在彎曲力。因此,

4、在大多數(shù)情況下 , 所有梁桁架橋是直的。桁架梁是由很多小桁架組成， 支撐大量的重量，跨越較大的距離。在大多數(shù)情況下 , 設(shè)計、制造和安裝桁架是 相對簡單的。然而，一旦組裝的桁架占用較大的空間，以復(fù)雜的結(jié)構(gòu)形成，就會 分散司機(jī)的注意力。像梁橋一樣， 既有簡單的桁架橋， 又有連續(xù)桁架橋。 桁架橋的各個部件體積 小，使其成為在大型部件或部分部件不能裝運或者大型起重機(jī)械和重型設(shè)備不能 使用的地方處理想的橋梁型式。 由于桁架是一個空洞的骨架結(jié)構(gòu)， 道路可能越過 （見插圖 2）甚至通過（見插圖 1），同時考慮到橋下的清場，這是其他橋型不能 比的。桁架也按基本設(shè)計使用來分類。 最具代表性的包括華倫桁架， 普

5、拉特桁架以 及豪威桁架。 華倫桁架也許是包括簡單或是連續(xù)桁架中最常見的型式。 對于跨度 較小的，無垂直桁架可以給結(jié)構(gòu)一個簡單的外觀（見插圖1）。對于跨度較大的桁架橋，垂直桁架可以提供額外的力（見插圖 2）。華倫結(jié)構(gòu)跨度通常為 50100 米。普拉特桁架因其對角線桁架組成除了兩邊均向跨度中心傾斜而得名。 除了靠 近中心的對角線桁架，其他所有的桁架只有在垂直短桁架受壓力時受到斜拉力。 這就使得可以用較為稀薄的桁架構(gòu)成使得設(shè)計更為經(jīng)濟(jì)。豪威桁架（見插圖 4）是普拉特桁架相反的結(jié)構(gòu)。對角線桁架朝向相反方向 并承擔(dān)壓力。這對于鋼橋來說是非常不經(jīng)濟(jì)的，因此它的使用也是很少見的。 剛架橋剛構(gòu)橋梁有時也被稱為

6、框架結(jié)構(gòu)的橋梁。 在標(biāo)準(zhǔn)的梁橋中， 主梁和橋墩是分 開的結(jié)構(gòu)。然而剛構(gòu)橋中主梁和橋墩是一個整體。剛構(gòu)橋的橫截面通常是 I 型或是箱型。連續(xù)剛構(gòu)橋的設(shè)計計算比簡支梁橋復(fù) 雜。橋墩和主梁的連接處制作困難，需要計算準(zhǔn)確且注重細(xì)節(jié)。盡管有許多可能的理論形狀，但是近些年來使用的結(jié)構(gòu)形式只有n型， 斜腿 形和 V 字型框架。斜腿剛構(gòu)橋特別適合于跨越河流山谷， 因為傾斜一定角度的橋墩可以有效的 跨越障礙而不需在河流中央修建基礎(chǔ)或在山谷中修建橋墩（見插圖1）。V型剛構(gòu)橋可以有效地利用基礎(chǔ)。每個 V字型墩提供兩個支撐梁的力，同時 減少基礎(chǔ)的數(shù)量，從而使得外形更加簡單。 （見插圖 3）n型剛構(gòu)結(jié)構(gòu)通常用于內(nèi)城高速

7、公路的橋墩或是支撐。該框架支撐起高出的公路，同時允許車輛從橋下直接通過。 （見插圖 2）拱橋在梁橋之后，拱橋是第二古老的橋式和經(jīng)典結(jié)構(gòu)。 不像簡支梁橋， 拱橋非常 適合用石材建造。 許多古老知名的拱橋時至今日仍然佇立著。 穿越山谷與河流拱 橋是個很好的選擇， 因為拱橋在中間部分不需要橋墩支撐。 拱橋可以成為更美橋 型之一。拱橋使用曲拱結(jié)構(gòu)， 提供一個高抗彎力。 不像梁橋和桁架橋， 拱橋的兩端固 定在同一水平方向上（即不允許水平方向運動存在于軸之中） 。因此當(dāng)負(fù)載在橋 上（如一輛車通過時） 水平力就會在拱的軸承中產(chǎn)生。 這些水平力對于拱橋來說 是獨特的，因此拱橋只能應(yīng)用于地面基礎(chǔ)牢固穩(wěn)定的地方。

8、就像桁架橋一樣，道路可能越過（見插圖 1）或者通過拱（見插圖 4）或者 在某種情況下，兩者都有（見插圖 3）。結(jié)構(gòu)上拱橋有四種基本結(jié)構(gòu)形式：無鉸 拱，雙鉸拱，三鉸拱和系桿拱橋。無鉸拱（見插圖 1）沒有鉸并且不允許基礎(chǔ)的轉(zhuǎn)動。因此基礎(chǔ)會承受大量的 荷載（水平，垂直，彎曲力） ，因此無鉸拱只能在地面基礎(chǔ)十分穩(wěn)定的情況下修 建。然而，無鉸拱是一種非常僵硬的結(jié)構(gòu)比其他拱產(chǎn)生的撓度也就相對少了。兩鉸拱（見插圖 2）使用了允許轉(zhuǎn)動的鉸接軸承。在軸承中只產(chǎn)生水平和垂 直力。這或許是最常用的鋼拱的更改，通常也是一個經(jīng)濟(jì)的設(shè)計方式。三鉸拱（見插圖 3）增加了一個鉸鏈在拱的頂部。 三鉸拱因基礎(chǔ)運動 （地震， 下沉

9、等）受到的影響較小。然而，三鉸拱會遭受等多的撓度影響，鉸鏈很復(fù)雜， 難以制作。因此三鉸拱很少使用。系桿拱橋（見插圖 4）是一種組合式拱橋，允許在地面不夠穩(wěn)固以抵抗水平 力的變化的情況下建造。 連接拱的兩端和本身而不是依賴基礎(chǔ)來承受水平力， 因 此稱之為系桿拱橋。斜拉橋一座典型的斜拉橋（見插圖 1和 2）是與一個或多個塔柱連續(xù)梁跨度中間橋 墩的架設(shè)。纜繩從這些塔柱上拉下（通常向兩邊）來支撐主梁。鋼纜繩非常強(qiáng)韌有彈性。 鋼纜繩非常經(jīng)濟(jì)， 且可以形成細(xì)長結(jié)構(gòu)但仍然能跨 越較遠(yuǎn)距離。 盡管少數(shù)的纜繩強(qiáng)度就足以支撐整個橋， 他們的彈性使得纜繩很難 承受我們很難考慮到得力：風(fēng)力。對于跨度較大的斜拉橋， 必

10、須做出仔細(xì)研究來確保纜繩和橋梁在風(fēng)力作用下 的穩(wěn)定情況。橋的重量越輕，對于風(fēng)力的抵抗能力越弱，但是對于抵抗地震卻是有利的。 但是，地震或是時間推移下地基的不均勻沉降會使斜拉橋產(chǎn)生破壞， 因此必須認(rèn) 真規(guī)劃基礎(chǔ)。斜拉橋即現(xiàn)代又簡潔的外觀，使之成為有吸引力獨特的地標(biāo)。纜繩的獨特屬性以及其作為一個整體結(jié)構(gòu)， 使得斜拉橋的設(shè)計十分復(fù)雜。 較 大跨度的斜拉橋， 風(fēng)力和溫度的影響必須考慮， 這個計算是非常復(fù)雜的， 不得不 借助計算機(jī)和計算機(jī)分析。 斜拉橋橋索的制作也比較困難。 橋索的布線和主塔的 附件構(gòu)造復(fù)雜，需要精密制造。斜拉橋沒有明顯的分類。但是，它可以根據(jù)跨越數(shù)，塔柱數(shù)，主梁類型和纜 繩數(shù)量區(qū)分。

11、塔柱的數(shù)量和類型有很多變化， 纜繩的數(shù)目和排列也有變化。 典型 的主塔形式有：單柱、雙柱、門形或是 A型（見插圖2和3）。橋索的排列也不盡相同。典型的型式有單面、扇形、豎琴型、星型（見插圖 4）。在某些情況下， 橋索只有在塔柱的一側(cè)安裝在梁上， 另一側(cè)被錨固在基礎(chǔ)上 或用以其他力來平衡。懸索橋現(xiàn)如今使用的所有橋型中，懸索橋是跨越距離最長的橋型。乍一看 , 這和懸 浮的斜拉橋可能看起來很相似 , 但他們有很大的不同。雖然大跨度懸索橋現(xiàn)如今 技術(shù)領(lǐng)先 , 但他們其實是一種很古老的形式的橋。一些最原始的懸索橋的例子是 使用藤蔓和繩子作為纜索。金屬的發(fā)展帶來了鐵棍和鎖鏈的使用。但是知道鋼絲索的引進(jìn)才使

12、得跨越 500 米以上成為現(xiàn)實。今天明石海峽大橋擁有世界最長的1991米的跨度。典型的懸索橋是與一個或多個橋塔通過纜繩與主梁在橋墩中央相連。 主梁本 身在較短的跨度下是桁架或是箱梁， 板梁并不少見。 橋的兩端通過放置大型錨固 設(shè)備或是重物用以固定纜繩。主纜延伸從一個錨錠越過塔頂連接到另一個錨錠處。 這種跨越結(jié)構(gòu)的纜繩稱 之為馬鞍（檢查圖 2）。馬鞍型允許纜繩從一邊到另一邊順利地傳遞荷載。從主纜懸掛下來的像掛鉤一樣的小纜繩懸掛下來連接在主梁上。 有些懸索橋 不適用錨錠，纜繩直接連接主梁的兩端。依靠這些自錨懸索橋跨度的重量 , 以平 衡中心跨度和錨定纜繩。因此，不同于普通橋梁依靠墩臺承重， 懸索橋

13、主梁或是橋面板實際上是懸掛 在主索上。橋的大部分重量和車輛荷載都被纜索承擔(dān)。 反過來，纜繩由主塔支撐， 因此主塔就必須支撐大量的重量。正如以上解釋的斜拉大橋鋼纜索強(qiáng)度極強(qiáng) , 然而靈活。就像一個很強(qiáng)韌的繩 子, 那將是很好的為懸掛或拉一些事的選擇 ,但是試圖推動物體是沒有用的。 大跨 度懸索橋盡管在普通交通荷載下很強(qiáng)韌， 極易受到風(fēng)荷載的作用。 必須采取特殊 的措施以確保大橋在強(qiáng)風(fēng)下不晃動或是振動。最著名的空氣動力不穩(wěn)定的大橋的例子是美國華盛頓州的塔科瑪灣海峽大 橋。這篇英國布里斯托爾大學(xué)對于塔科瑪灣海峽大橋災(zāi)難的優(yōu)秀的網(wǎng)頁照片和短 片會解釋為什么空氣動力穩(wěn)定性如此重要。BridgesGird

14、er BridgeA girder bridge is perhaps the most com mon and most basic bridge. A log across a creek is an example of a girder bridge in its simplest form. In moder nsteel girder bridges, the two most com mon girders are I-beam girders and box-girders.FlangeFlangeWtbIf we look at the cross secti on of a

15、n I-beam girder we can immediately understand why it is called an Ibeam (illustrati on #1.) The cross sect ion of the girder takes the shape of the capital letter I. The verticalplate in the middle is known as the web, and the top and bottom plates are referred to as flan ges. To expla in why the I

16、shape is an efficie nt shape for a girder is a long and difficult task so we won't attempt that here.Typ cal Span Lengths1 Dm - 200mWorld's LongestPonte Costa a Silva. Braz ITotal LengthCenler Span7OOrn300mA Matsuo ExampleA box girder is much the sameFlange即亦-FimgEHowever, i n some cases the

17、re are more tha n two webs, creati ng a multipleas an I-beam girder except that, obviously, it takes the shape of a box. The typical box girder has two webs and two flan ges (illustrati on #2.)chamber box girder.Other examples of simple girders in clude pi girders, n amed for their like ness to the

18、mathematical symbol for pi, and T shaped girders. Since the majority of girder bridges these days are built with box or I-beam girders we will skip the specifics of these rarer cases.Now that we know the basic physical differe nces betwee n box girders and I-beam girders, let's look at the adva

19、ntages and disadva ntages of each. AnI-beam is very simple to desig n and build and works very well in most cases. However, if the bridge contains any curves, the beams become subject to twist ing forces, also known as torque. The added sec ond web in a box girder adds stability and in creases resis

20、ta nee to twist ing forces. This makes the box girder the ideal choice for bridges with any significant curve in them.Box girders, being more stable are also able to spa n greater dista nces and are ofte n used for Ion ger spa ns, where I-beams would not be sufficie ntly strong or stable. However, t

21、he design and fabrication of box girders is more difficult than that of I beams. For example, in order to weld the in side seams of a box girder, a huma n or weld ing robot must be able to operate in side the box girder.TrussThe truss is a simple skeletal structure. I n desig n theory, the in dividu

22、al members of a simple truss are on ly subject to tension and compressi on forcesand not bending forces.Thus, for the most part, all beams in a truss bridge are straight. Trusses areTypical Span Lengths40m - 500mWorld's LongestPont de QuebecTota. Length863mCenter Span549mA Matsuo Exannp 92nd M a

23、mevakl Brldae/WVWcomprised of many small beams that together can support a large amount of weight and spa n great dista nces. In most cases the desig n, fabricati on, and erecti on of trusses is relatively simple. However, once assembled trusses take up a greater amount of space an d, i n more compl

24、ex structures, can serve as a distracti on to drivers.Like the girder bridges, there are both simpleand con ti nu ous trusses. The small size of in dividual parts of a truss make it the ideal bridge for places where large parts or secti ons cannot be shipped or where large cranes and heavy equipme n

25、t cannot be used duri ng erecti on. Because the truss is a hollow skeletal structure, the roadway may pass over (illustrati on #2) or eve n through (illustrati on #1) the structure allow ing for cleara nee below the bridge ofte n not possible with other bridge types. Trusses are also classified by t

26、he basic desig n used. The most represe ntative trusses are the Warren truss, the Pratt truss, and the Howe truss. The Warre n truss is perhaps the most com mon truss for both simple and continu ous trusses. For smaller spa ns, no vertical members are used lending the structure a simple look (illust

27、rati on #1.) For Ion ger spa ns vertical members are added provid ing extra stre ngth (illustrati on #2.) Warre n trusses are typically used in spa ns of betwee n 50-100m.The Pratt truss (illustrati on #3) is ide ntified by its diago nal members which, except for the very end on es, all sla nt dow n

28、 and in toward the cen ter of the spa n. Except for those diag onal members n ear the cen ter, all the diag onal members are subject to tension forces only while the shorter vertical members han dle the compressive forces. This allows for thinner diag onal members result ing in a more econo mic desi

29、g n. The Howe truss (illustrati on #4) is the opposite of the Pratt truss. The diagonal members face in the opposite directi on and han dle compressive forces. This makes it very uneconomic design for steel bridges and its use is rarely see n.Rigid FrameRigid frame bridges are sometimes also known a

30、s Rahme n bridges. In a sta ndard girder bridge, the girder and the piers are separate structures. However, a rigid frame bridge is one in which the piers and girder are one solid structure.The cross secti ons of the beams in a rigid frame bridge are usually I shaped or box shaped. Design calculatio

31、ns for rigid frame bridges are more difficult than those of simple girder bridges. The junction of the pier and the girder can be difficult to fabricate and requires accuracy and atte nti on to detail.Though there are many possible shapes, the styles used almost exclusively these days are the pi-sha

32、ped frame, the batter post frame, and the V shapedframe.The batter post rigid frame bridge is particularly well suited for river and valley crossings because piers tilted at an an gle can straddle the cross ing moreeffectively without requiring the construction of foundations in the middle of theriv

33、er or piers in deep parts of a valley (illustratio n #1).V shaped frames make effective use of foun dati ons. Each V-shaped pier provides two supports to the girder, reduc ing the nu mber of foun dati ons and creati ng a less cluttered profile (illustrati on #3.)frequently as the piers and supports

34、for inner city highways. The frame supports the raised highway and at the same time allows traffic to run directly under the bridge (illustrati on #2.)Pi shaped rigid frame structures are usedArchAfter girders, arches are the sec ond oldest bridge type and a classic structure. Un like simple girder

35、bridges, arches are well suited to the use of stone. Many an cie nt and well know examples of stone arches still sta nd to this day. Arches are good choices for cross ing valleys and rivers since the arch does n't require piers in the center. Arches can be one of the more beautiful bridge types.

36、Tiraispan l$ 葦40m Woitn Longaitd River日丐吏 u.SATotal lengthW4rnC*ni*r 辟an5i BmA MgSsuoEMamip-eArches use a curved structure which provides a high resista nee to bending forces. Un like girder and truss bridges, both ends of an arch are fixed in the horiz on tal directi on (i.e. no horiz on tal moveme

37、 nt is allowed in the beari ng). Thus whe n a load is placed on the bridge (e.g. a car passes over it) horiz on tal forces occur in the beari ngs of the arch. These horiz on tal forces are unique to the arch and as a result arches can only be used where the ground or foun dati on is solid and stable

38、Like the truss, the roadway may pass over (illustratio n #1) or through an arch (illustrati on #4) or in some cases both (illustrati on #3.) Structurally there are four basic arch types: hin ge-less, two-h in ged, three hin ged and tied arches.bending forces) and the hin ge-less arch c very stable.

39、However, the hin ge-less arch less deflect ion tha n other arches.The hin ge-less arch (illustratio n #1) uses no hin ges and allows no rotati on at the foun dati ons. As a result a great deal of force is gen erated at the foun dati on (horiz on tal, vertical, and 汨 only be built where the ground is

40、 is a very stiff structure and suffersThe two hin ged arch (illustrati on #2) uses hin ged beari ngs which allow rotati on.The only forces gen erated at the beari ngs are horiz on tal and vertical forces. This is perhaps the most com monly used variati on for steel arches and is gen erally a very ec

41、ono mical desig n.The three-hinged arch (illustration #3) adds an additi onal hinge at the top or crow n of the arch. The three-h in ged arch suffers very little if there is moveme nt in either foun dati on (due toearthquakes, sinking, etc.) However, the three-hi nged arch experie nces much more def

42、lecti on and the hin ges are complex and can be difficult to fabricate.The three-hi nged arch is rarely used anymore.The tied arch (illustrati on #4) is a variati on on the arch which allowscon struct ion eve n if the ground is not solid eno ugh to deal with the horiz on tal forces. Rather tha n rel

43、 ying on the foun dati on to restrain the horiz on tal forces, the girderitself "ties" both ends of the arch together, thus the name "tied arch."Cable StayedA typical cable stayed bridge (illustratio n #1 & 2) is a con ti nu ous girder withone or more towers erected above pie

44、rs in the middle of the spa n. From thesetowers, cables stretch dow n diag on ally (usually to both sides) and support thegirder.Steel cables are extremely stro ng but very flexible. Cables are very econo mical as they allow a slender and lighter structure which is still able to spa n great dista nc

45、es.Though only a few cables are strong eno ugh to support the en tire bridge, theirflexibility makes them weak to a force we rarely con sider: the wind.guara ntee the stability of the cables and the bridge in the wind.Typical Span L«ngth>113m-4E0mWorW's Lowest Tatara Bridge. Japan Total

46、Length ' ,430m Center Span 890mA Matiuo Exampl*Fsnrunri： Tsobasa 日riciqEFor Ion ger spa n cable-stayed bridges, careful studies must be made toThe lighter weight of the bridge, though a disadva ntage in a heavy win d, is an adva ntage duri ng an earthquake. However, should un eve n settl ing of

47、the foun dati ons occur duri ng an earthquake or over time, the cable-stayed bridge can suffer damage so care must be take n in pla nning the foun datio ns. The moder n yet simple appeara nee of the cable-stayed bridge makes it an attractive and dist inct Ian dmark.The unique properties of cables, a

48、nd the structure as a whole, make the desig n of the bridge a very complex task. For Ion ger spa ns where winds andtemperatures must be con sidered, the calculati ons are extremely complex and would be virtually impossible without the aid of computers and computer analysis. The fabrication of cable

49、stay bridges is also relatively difficult. The cable routi ng and attachme nts for the girders and towers are complex structures requiri ng precisi on fabricati on.There are no dist inct classificati ons for cable-stayed bridges. However, theyPorudAShapedcan disti nguished by the nu mber of spa ns,

50、nu mber of towers, girder type, nu mber of cables, etc. There are many variati ons in the nu mber and type of towers, as well as the nu mber and arran geme nt of cables. Typical towers used aresin gle, double, portal, or eve n A-shaped towers (illustrati on #2 & 3.)ITCable arran geme nts also va

51、ry greatly. Some typical varieties are mono, harp, fan, and star arran geme nts (illustrati on #4.) In some cases, on ly the cables on one side of the tower are attached to the girder, the other side being an chored to a foun dati on or other coun terweight.Typica? Span Lengths7 Dm - 1,000m+Wond'

52、;-s LongestAkashi Kalkyc Bridge. JapanTotal Length3.911mCenter Span1.591mA Matsuo ExampleHakucho BridaeOhnaruto Brlda甘SuspensionOf all the bridge types in use today, the suspe nsion bridge allows for the Ion gest spa ns. At first gla nce the suspe nsion and cable-stayed bridges may look similar, but

53、 they are quite differe nt. Though suspe nsion bridges are leadi ng long spa n tech no logy today, theyare in fact a very old form of bridge. Some primitive examples of suspension bridges use vines and ropes for cables.The developme nt of metals brought the use of lin ked iron bars and cha ins. But

54、it was the in troduct ion of steel wire ropes that allowed spa ns of over 500m to become a reality. Today the Akashi Kaikyo bridge boasts the world's Ion gest cen ter spa n of any bridge at 1,991 meters.A typical suspe nsion bridge (illustrati on #1) is a con ti nu ous girder with one or more towers erected above piers in the middle of the span. The girder itself it usually a truss or box girder though in shorter spa ns, plate girders are not un com mon. At both ends of the bridge large an chors or coun ter weights are placed to hold the ends of the